1. Field of the Invention
The present invention relates to apparatus for transforming with high diffraction efficiency a single frequency, linearly polarized laser beam into a beam with two frequency components that are collinear and orthogonally polarized. More particularly, the invention relates to electro-optical apparatus that are useful in a variety of optical measuring devices that perform extremely accurate measurement of changes in either length or optical length.
2. The Prior Art
The use of optical interferometry to measure changes in either length, distance, or optical length has grown significantly due not only to technological advances in lasers, photosensors, and microelectronics but also to an ever increasing demand for high precision, high accuracy measurements (cf. N. Bobroff, "Recent advances in displacement measuring interferometry," Meas. Sci. Technol. Vol. 4, pp. 907-926 (September 1993)). The prior art interferometers can be generally categorized into two types based on the signal processing technique used, i.e., either homodyne or heterodyne. The interferometers based on the heterodyne technique are generally preferred because (1) they are insensitive to low frequency drift and noise and (2) they can more readily have their resolution extended. Within the heterodyne type of interferometers, of particular interest are the ones based on the use of two optical frequencies. In the prior art two-optical frequency heterodyne interferometers, the two optical frequencies are produced by one of the following techniques: (1) use of a Zeeman split laser, see for example, Bagley et al., U.S. Pat. No. 3,458,259, issued Jul. 29, 1969; G. Bouwhuis, "Interferometrie Mit Gaslasers," Ned. T. Natuurk, Vol. 34, pp. 225-232 (August 1968); Bagley et al., U.S. Pat. No. 3,656,853, issued Apr. 18, 1972; and H. Matsumoto, "Recent interferometric measurements using stabilized lasers," Precision Engineering, Vol. 6, pp. 87-94 (1984); (2) use of a pair of acousto-optic Bragg cells, see for example, Y. Ohtsuka and K. Itoh, "Two-frequency Laser Interferometer for Small Displacement Measurements in a Low Frequency Range," Applied Optics, Vol. 18, pp. 219-224 (1979); N. Massie et al., "Measuring Laser Flow Fields With a 64-Channel Heterodyne Interferometer," Applied Optics, Vol. 22, pp. 2141-2151 (1983); Y. Ohtsuka and M. Tsubokawa, "Dynamic Two-frequency Interferometry for Small Displacement Measurements," Optics and Laser Technology, Vol. 16, pp. 25-29 (1984); H. Matsumoto, op. cit.; P. Dirksen, et al., U.S. Pat. No. 5,485,272, issued Jan. 16, 1996; N. A. Riza and M. M. K. Howlader, "Acousto-optic system for the generation and control of tunable low-frequency signals," Opt. Eng., Vol. 35, pp. 920-025 (1996); (3) use of a single acousto-optic Bragg cell, see for example, G. E. Sommargren, commonly owned U.S. Pat. No. 4,684,828, issued Aug. 4, 1987; G. E. Sommargren, commonly owned U.S. Pat. No. 4,687,958, issued Aug. 18, 1987; P. Dirksen, et al., op. cit.; or (4) use of two longitudinal modes of a randomly polarized HeNe laser, see for example, J. B. Ferguson and R. H. Morris, "Single Mode Collapse in 6328 .ANG. HeNe Lasers," Applied Optics, Vol. 17, pp. 2924-2929 (1978).
As for the prior art use of a Zeeman split laser to produce the two optical frequencies, this approach is only applicable to certain lasers (e.g., HeNe) and limits the frequency difference between the two optical frequencies to about 2 MHz. This imposes a limit on the maximum rate of change of the length or optical length being measured. In addition, the available power from a Zeeman spilt laser is less than 500 microwatts which can be a serious limitation when one laser source must be used for the measurement of multiple axes, such as three to six axes.
As for the prior art use of a single Bragg cell in the commonly owned Sommargren, U.S. Pat. No. 4,687,958, the diffraction efficiency may be low because the optic axis of the uniaxial crystal, the direction of the input optical beam, and the direction of the acoustic beam, are approximately collinear and under these conditions, a small change in the direction of the input beam such as caused by diffraction, will result in an unacceptable momentum mismatch. Also in commonly owned Sommargren, U.S. Pat. No. 4,687,958, the diffraction efficiency may be low for a number of different types of uniaxial crystals because the efficiency of the dominant Bragg diffraction mode in this group of uniaxial crystals is proportional to the sine of the angle between the optic axis of the uniaxial crystal and either one or the other of the directions of the optical beam components or the direction of the acoustic beam. To compensate for the possibility of low diffraction efficiency resulting from the latter of these two reasons, the path length in the Bragg cell may be increased. However, this procedure may lead to a Bragg cell that is inordinately long which in turn can result in an expensive apparatus. Also an extended length may lead to a diffracted beam with width elongated in the plane of diffraction and an increased lateral separation also in the diffraction plane between orthogonally polarized beam components. These two potential low diffraction efficiency problems are not encountered in the present invention because the optic axis of the uniaxial crystal and the direction of the acoustic beam are approximately orthogonal. In addition, if the uniaxial crystal in the Bragg cell of commonly owned U.S. Pat. No. 4,687,958 is optically active (e.g., quartz), there may be an accompanying reduction in the diffraction efficiency which subsequently requires a further increase in the required path length for what is already an expensive apparatus. The angle between the optic axis and the acoustic beam can be increased in commonly owned Sommargren, U.S. Pat. No. 4,687,958 in order to achieve a reduction of the momentum mismatch problem and of the required path length. However, this angle can preferably only be increased by approximately 27 degrees because for larger angles, the orthogonally polarized beam components may suffer total internal reflection at the nominal exit face of the Bragg cell and the device would cease to function.
As for the prior art use of a single Bragg cell in commonly owned Sommargren, U.S. Pat. No. 4,684,828, the apparatus has many parts which are separated and require more space, is sensitive to misalignment of the various parts, is more sensitive to thermal gradients in the apparatus as a result of multiple parts and the required separation of multiple parts, and is not as efficient as the present invention, i.e., approximately 50% of the input beam intensity is transformed into the output beam with the commonly owned Sommargren, U.S. Pat. No. 4,684,828 as compared to nominally 100% with the apparatus described herein.
As for the prior art use of a single Bragg cell in Dirksen, et al., op. cit., the frequency difference between the two orthogonally polarized components of the exit beam is twice the frequency of the acoustic beam. Starting with a higher frequency difference, the task of resolution extension becomes more difficult and expensive. Also with the single Bragg cell in Dirksen, et al., op. cit., the efficiency is limited to approximately 80% and there are significant non uniform intensity distributions across the two orthogonally polarized exit beam components in contrast to the present invention described herein. These non uniform intensity distributions across the width of the two orthogonally polarized exit beam components have a negative cross-correlation coefficient which further exacerbates the effect of non uniform beam component intensities for interferometry. There is generally polarization mixing in both of the two exit beam components since Dirksen et al., op. cit. uses normal Bragg diffraction mode which limits its utility in precision interferometric measurements. The apparatus of Dirksen et al., op. cit. is more complex requiring a minimum of two optical elements in addition to the Bragg cell and generates two desired and two undesired beam components which must be separated by external masks to occult the undesired beam components. There is more sensitivity to changes in temperature and temperature gradients because of multiple elements comprised of different materials and because of the larger angles between the components of the beams inside Dirksen et al., op. cit., apparatus in contrast to that found in the apparatus of the present invention described herein. The angles between the components of the beams inside Dirksen et al. op. cit. apparatus are generally larger by design because Dirksen et al., op. cit., must spatially separate the desired and undesired beam components as a result of using normal Bragg diffraction in contrast to the present invention described herein which uses anisotropic Bragg diffraction. The Dirksen et al., op. cit., apparatus, which requires separation of parts, is sensitive to misalignment with additional sensitivity to thermal gradients. In addition, since the acoustic frequency for a single Bragg cell apparatus in Dirksen et al., op. cit., is one half of the frequency difference between the two orthogonally polarized components of the exit beam, the apparatus in Dirksen et al., op. cit. must operate at a lower acoustical frequency by a factor of two than in the apparatus of the present invention described herein to obtain a given frequency difference between the two orthogonally polarized components of the exit beam which leads to an additional reduction in efficiency.
As for the prior art use of two Bragg cells in Dirksen et al., op. cit., the apparatus has more parts than the single Bragg cell apparatus described in Dirksen et al., op. cit., which are well separated and require more space, there is generally polarization mixing in each of two exit beam components since the Dirksen et al., op. cit., uses normal Bragg diffraction mode, is sensitive to misalignment of the various parts, is more sensitive to thermal gradients in the apparatus as a result of multiple parts and the required separation of multiple parts, is not as efficient as the apparatus of the present invention described herein, i.e., approximately 60% to 80% of the input beam intensity is transformed into the output beam with the Dirksen et al., op. cit., as compared to nominally 100% with the apparatus described herein, generates two desired and either two or six undesired exit beam components depending on apparatus configuration selected, increased non uniform intensity distributions across two orthogonally polarized exit beam components, non uniform intensity distributions across widths of two orthogonally polarized exit beam components have negative cross-correlation coefficient which further exacerbates the effect of non uniform beam component intensities, and external masks are required to occult undesired beam components.
Finally, although the prior art use of two longitudinal modes of a randomly polarized HeNe laser provides a laser beam with two orthogonally polarized frequencies in a rather convenient, cost-effective form, the frequency difference is approximately 500-600 MHz which requires complicated, expensive detection and processing electronics. Furthermore, by starting out with such a high frequency difference, the task of resolution extension becomes difficult and expensive.
Some of the principal advantages of the present invention are that the apparatus: is comprised of a single crystal with piezoelectric transducer bonded directly to the crystal, has high diffraction efficiency, has compact size requiring no external beam stops, has an efficiency of nominally 100% for conversion of input intensity into intensities of two orthogonally polarized exit beam components, and the intensity of each of two orthogonally polarized exit beam components may be adjusted to nominally 50% of the input intensity. In addition, the apparatus of the present invention can generate in a single Bragg diffraction two orthogonally polarized exit beam components with a frequency difference well below the critical acoustical frequency of the Bragg cell. The appartus of the present invention also exhibits reduced polarization mixing, has more uniform intensity distributions across two orthogonally polarized exit beam components, has reduced lateral displacement between orthogonally polarized exit beam components because of anisotropic Bragg diffraction used in the present invention, and has lateral displacement between the orthogonally polarized exit beam components not effected by misalignment of multiple elements. Furthermore, the apparatus of the present invention has reduced angular displacement between orthogonally polarized beam components inside the Bragg cell which is made possible by the use of anisotropic Bragg diffraction in the present invention, and the angular displacement between orthogonally polarized exit beam components has reduced sensitivity to changes in temperature or to temperature gradients of the apparatus in part because of reduced angular displacement between orthogonally polarized beam components inside the Bragg cell. The angular displacement between input and exit beam in the apparatus of the present invention has reduced sensitivity to changes in temperature or to temperature gradients of apparatus, and, futhermore, there are not multiple elements to create misalignment of directions of the input and exit beam.
Consequently, while prior art techniques for producing a laser beam with two optical frequencies of orthogonal polarization are useful for some applications, none known to applicant provide the technical performance in a commercially viable form for applications requiring the measurement of rapidly changing lengths (distances) to extremely high resolution. The aforementioned disadvantages of the prior art are overcome by the present invention.